CONCRETE

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What is Concrete?

Concrete is one of the most commonly used building materials.

Concrete is a composite material made from several readily available constituents (aggregates, sand, cement, water).

Concrete is a versatile material that can easily be mixed to meet a variety of special needs and formed to virtually any shape.

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Advantages

Ability to be cast Economical Durable Fire resistant Energy efficient On-site fabrication

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Disadvantages

Low tensile strength Low ductility Volume instability Low strength to weight ratio

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Constituents

- Cement- Water - Fine aggregates- Coarse aggregates - Admixtures chemicals which are added to concrete at the

mixing stage to modify some of the properties.

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PROPERTIES OF FRESH CONCRETE

Workability Consistency Segregation Bleeding Setting Time Unit Weight Uniformity

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WORKABILITY

It is desirable that freshly mixed concrete be relatively easy to transport, place, compact and finish without harmful segregation. A concrete mix satisfying these conditions is said to be workable.

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Factors Affecting Workability

Method and duration of transportation Quantity and characteristics of

cementing materials Aggregate grading, shape and surface

texture Quantity and characteristics of

chemical admixtures Amount of water Amount of entrained air Concrete & ambient air temperature

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WORKABILITY

Workability is the most important property of freshly mixed concrete.

There is no single test method that can simultaneously measure all the properties involved in workability.

It is determined to a large extent by measuring the “consistency” of the mix.

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CONSISTENCY

Consistency is the fluidity or degree of wetness of concrete.

It is generally dependent on the shear resistance of the mass.

It is a major factor in indicating the workability of freshly mixed concrete.

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CONSISTENCY

Test methods for measuring consistency are:

Flow test → measures the amount of flow Kelly-Ball test → measures the amount of

penetration Slump test (Most widely used test!)

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Slump Test is related with the ease with which concrete flows during placement (TS 2871, ASTM C 143), TS- Technical specification

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10 cm

20 cm

30 cm

The slump cone is filled in 3 layers. Every layer is evenly rodded 25 times.

Measure the slump by determining the vertical difference between the top of the mold and the displaced original center of the top surface of the specimen.

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SEGREGATION Segregation refers to a separation of the

components of fresh concrete, resulting in a non-uniform mix

  Sp.Gr. Size

Cement 3-3.15

5-80 mm

C.Agg.2.4-

2.85-40

mm

F.Agg.2.4-

2.8 < 5 mm

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The primary causes of segregation are

• differences in specific gravity and size of constituents of concrete. • improper mixing and

improper placing.• Particle shape and texture.• Reduced Water/cement

ratio. • improper consolidation

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BLEEDING Bleeding is the tendency of water to

rise to the surface of freshly placed concrete.

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It is caused by the inability of solid constituents of the mix to hold all of the mixing water as they settle down.

A special case of segregation.

BLEEDINGUndesirable effects of bleeding are:• With the movement of water towards the top, the top portion

becomes weak & porous (high w/c). Thus the resistance of concrete to freezing-thawing decreases.

when water seeps into a crack in a rock, as the temperature drops below freezing, the water freezes and expands causing the crack to enlarge. The ice then melts into water again as the temperature rises above 0 degrees C. This action is repeated until the rock breaks.

• Water rising to the surface carry fine particles of cement which weaken the top portion and form laitance (weak layer of fine aggregate and cement). This portion is not resistant to abrasion.

• Water may accumulate under the coarse agg. and reinforcement. These large voids under the particles may lead to weak zones and reduce the bond between paste and agg. or paste and reinforcement.

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BLEEDING

The tendency of concrete to bleeding depends largely on properties of cement. It is decreased by:

Increasing the fineness of cement Increasing the rate of hydration (C3S,

C3A and alkalies) Adding pozzolans (siliceous or siliceous

and aluminous material which involve hydration reaction)

Reducing water content 19

MIXING OF CONCRETE

The aim of mixing is to blend all of the ingredients of the concrete to form a uniform mass and to coat the surface of aggregates with cement paste.

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MIXING OF CONCRETE Ready-Mix concrete: In this type ingredients

are introduced into a mixer truck and mixed during transportation to the site. • Wet – Water added before transportation• Dry – Water added at site

Mixing at the site• Hand mixed• Mixer mixed

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Ready Mix Concrete

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Mixing at Site

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MIXING OF CONCRETEMixing time should be sufficient to produce a uniform concrete. The time of mixing depends on the type of mixer and also to some properties of fresh concrete.

Undermixing → non-homogeneity Overmixing → danger of water loss,

brekage of aggregate particles

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CONSOLIDATING (properly mixed) CONCRETE

Inadequate consolidation can result in:– Honeycomb– Excessive amount of entrapped air voids

(bug holes)– Sand streaks (unmixed fine sand and cement make

long thin strips on the surface)

– Placement lines (Cold joints)

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Honeycombing due to poor consolidation

Cold joints are created when concrete is poured against concrete that has already hardened to some degree. This condition results in a weakened bond between the existing and the newly poured concrete

VIBRATION OF CONCRETE

The process of compacting concrete consists essentially of the elimination of entrapped air. This can be achieved by:

– Tamping or rodding the concrete– Use of vibrators

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CURING OF CONCRETE Properties of concrete can improve with

age as long as conditions are favorable for the continued hydration of cement. These improvements are rapid at early ages and continues slowly for an indefinite period of time.

Curing is the procedures used for promoting the hydration of cement and control of temperature and the moisture movement from and into the concrete.

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CURING OF CONCRETE

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The primary objective of curing is to keep concrete saturated or as nearly saturated as possible.

Curing Methods1. Methods which supply additional water to the

surface of concrete during early hardening stages.– Using wet covers– Straw or hay – Ponding

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Curing Methods

2. Methods that prevent loss of moisture from concrete by sealing the surface.– Water proof plastics– Use liquid membrane (thin layer)-forming

compounds

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Curing Methods

3. Methods that accelerate strength gain by supplying heat & moisture to the concrete.

– By using live steam (steam curing)– Heating coils.

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PROPERTIES OF HARDENED CONCRETE

The principal properties of hardened concrete which are of practical importance can be listed as:1. High Strength2. Low Permeability & high durability3. Low Shrinkage & high creep resistance4. Positively response to temperature variations

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compressive strength is one of the most important property that is often required, simply because;

1. Concrete is used for compressive loads2. Compressive strength is easily obtained3. It is a good measure of all the other properties.

What Affects Concrete Strength

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What Doesn’t?

Factors Affecting Strength Effect of materials and mix proportions Production methods

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The strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on:1. w/c ratio2. Degree of compaction

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COMPRESSIVE STRENGTH Compressive Strength is determined

by loading properly prepared and cured cubic, cylindrical or prismatic specimens under compression.

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Cubic specimens without capping

Cylindrical specimens

with capping

Cubic: 15x15x15 cmCylinder: h/D=2 with h=15

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Bonded sulphur capping Unbonded neoprene pads40

TENSILE STRENGTH

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Tensile Strength can be obtained either by direct methods or indirect methods.

Direct methods suffer from a number of difficulties related to holding the specimen properly in the testing machine without introducing stress concentration and to the application of load without eccentricity.

DIRECT TENSILE STRENGTH

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SPLIT TENSILE STRENGTH

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Due to applied compression load a fairly uniform tensile stress is induced over nearly 2/3 of the diameter of the cylinder perpendicular to the direction of load application.

The advantage of the splitting test over the direct tensile test is the same molds are used for compressive & tensile strength determination.

The test is simple to perform and gives uniform results than other tension tests.

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σst = 2PπDl

P: applied compressive load

D: diameter of specimen

l: length of specimen

Splitting Tensile

Strength

FLEXURAL STRENGTH

The flexural tensile strength at failure or the modulus of rupture is determined by loading a prismatic concrete beam specimen.

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The results obtained are useful because concrete is subjected to flexural loads more often than it is subjected to tensile loads.

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P

M=Pl/4

d

b

cI =

bd3

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σ =M cI

=(Pl/4) (d/2)

bd3/12=

Plbd2

M=Pl/6

P/2 P/2

σ =(Pl/6) (d/2)

bd3/12

=Plbd2

2nd moment of Area, I

Bending moment, M

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Factors Affecting the Strength of Concrete

1) Factors depended on the test type:

– Size of specimen– Size of specimen in

relation with size of agg.

– Support condition af specimen

– Moisture condition of specimen

– Type of loading adopted– Rate of loading– Type of test machine

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2. Factors independent of test type:

– Type of cement– Type of agg.– Degree of compaction– Mix proportions– Type of curing– Type of stress situation

STRESS-STRAIN RELATIONS IN CONCRETE

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σult

(40-50%) σult

εult

σ-ε relationship for concrete is nonlinear. However, specially for cylindrical specimens with h/D=2, it can be assumed as linear upto 40-50% of σult

MODULUS OF ELASTICITY OF CONCRETE

Due to the nonlinearity of the σ-ε diagram, E is the defined by:1. Initial Tangent

Method2. Tangent Method3. Secant Method

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PERMEABILITY OF CONCRETE Permeability is important because:

1. The penetration of some aggresive solution may result in leaching out of Ca(OH)2 which adversely affects the durability of concrete.2. In reinforced concrete ingress of moisture of air

into concrete causes corrosion of reinforcement and results in the volume expansion of steel bars, consequently causing cracks & spalling of concrete cover.

3. The moisture penetration depends on permeability & if concrete becomes saturated it is more liable to frost- action. Frost action is a type of weathering where water actually seeps into cracks in the rock. The water freezes as the temperature drops and expands. This expansion creates a wedge inside the rock, encouraging split apart.

4. In some structural members permeability itself is of importance, such as, dams, water retaining tanks.

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PERMEABILITY OF CONCRETE

The permeability of concrete is controlled by capillary pores. The permeability depends mostly on

- w/c, - age - degree of hydration.

In general the higher the strength of cement paste, the higher is the durability & the lower is the permeability.

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DURABILITY

A durable concrete is the one which will withstand the effects of service conditions to which it will be subjected in a satisfactory degreeFactors Affecting Durability:

External → Environmental Internal → Permeability, Characteristics of

ingredients, Air-Void System... 54

Common Sources for Raw Materials

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Lime (CaO) - Limestone, shale Silica (SiO2) -Clay, sand, shale Alumina (Al2O3) - Clay, fly ash, shale

Iron (Fe2O3) - Clay, iron ore

Main Compounds in Cement Clinker

Tri Calcium Silicate (C3S)-3CaO.SiO2

Di Calcium Silicate (C2S)- 2CaO.SiO2

Tri Calcium Aluminate (C3A)- 3CaO.Al2O3

Tetra Calcium Alumino Ferrite (C4AF)- 4CaO.Al2O3.Fe2O3

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Tri Calcium Silicate (C3S)

Mainly responsible for early strength development.

Normal Portland Cement kiln feed has a C3S potential of 52-62%

Kiln feed with a potential in excess 65% is;

Difficult to burnHas poor coating facilities

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Di Calcium Silicate (C2S)

Account for approximately 22% of clinker.

Responsible for strength development, however very slow compared to C3S.

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Tri Calcium Aluminate (C3A)

Responsible for the workability of mortar. In normal portland cement C3A amounts to

6 to 8% of the clinker. Concretes containing cement high in C3A is

not resistant to attack by sulphates in soil or water.

The reason is SO4 – ions attack C3A to form an expanding compound name ‘Ettringite’.

SO4 – + C3A 3CaO.3CaSO4.30H2O

Ettringite

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Tetra Calcium Alumino Ferrite (C4AF)

Governs the colour of cement.

Higher the content of C4AF in the clinker the darker the cement.

During the burning iron acts as a fluxing agent.

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Minor Compounds

MgO -Acts as a fluxer.

-Excess MgO leads to unsound concrete.Sound concrete will not crack under excessive usage.free of structural flaws, which is absolutely relevant to standards of concrete.

Alkalines (Na2O,K2O)– Content less than 1%– Leads to internal expansion and failure of

concrete.61

Reaction Temperatures within the kiln

1000C- Evaporation of free water from the feed.

5000C- Evolution of combined water from clay.

8000C- Start of calcination (removal of a volatile fraction).

800-9000C- Formation of Di Cacium Silicate.

1100-12000C- Formation of C3A and C4AF.

1260-14500C- Formation of C3S.

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Reaction of OPC on addition of water During the hydrolysis the tri calcium silicate

(C3S) and di calcium silicate (C2S) in ordinary Portland cement reacts with water and produces a porous solid which may be defined as rigid gel referred as “Tobomorite” :Calcium Silicate Hydrate (CSH)

2 (3CaO. SiO2)+ 6 H2O 3 CaO. 2SiO2. 3H2O + 3 Ca(OH)2

2 (2CaO. SiO2)+ 4 H2O 3 CaO. 2SiO2. 3H2O + Ca(OH)2

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Leaching & Efflorescence When water penetrates into concrete, it dissolves

the non-hydraulic CH and various salts, sulfates and carbonates of Na, K, Ca.

These salts are taken outside of concrete by water and leave a salt deposit.

surface of concrete causing a change in appearance.

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Sulfate Attack

Ground water in soils containing alkali sulfates may affect concrete.

These solutions attack CH to produce gypsum. Later, gypsum and calcium alumina sulfates together with water react to form “ettringite”.

Formation of ettringite is hardened cement paste or concrete leads to volume expansion thus cracking.

Moreover, Magnesium sulfate may lead to the decomposition of the C-S-H gel.

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Sulfate Attack

Seawater contains some amount of Na and Mg Sulfates. However, these sulfates do not cause severe deleterious expansion/cracking because both gypsum and ettringite are soluble in solutions containing the Cl ion. However, problem with seawater is the frequent wetting/drying and corrosion of reinforcing steel in concrete.

To reduce the sulfate attack1. Use low w/c ratio→ reduced permeability & porosity

2. Use proper cement → reduced C3A and C3S 3. Use pozzolans → they use up some of the CH to

produce C-S-H

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